Expose EditorUndoRedoManager's clear_history()

[KoBeWi]

Exposes clear_history() method (I also swapped its arguments, the new order makes more sense).
Closes godotengine/godot-proposals#2153

I also exposed EditorUndoRedoManager via EditorInterface, to make it accessible to EditorScripts.

[KoBeWi]

I just realized we have EditorInterface.mark_scene_as_unsaved() 🙃
The new method is more general, but it makes the old one redundant.

EDIT:
Maybe not completely redundant, as that method adds a generic dirty marker and doesn't clear history.

[Mickeon]

I don't really see the benefit of this at the moment, but it's not exactly "harmful" either.

[KoBeWi]

Clearing history is necessary for operations that do changes to the scene which you can't undo them. Also exposing EditorUndoRedoManager is needed for things like #78355 (currently you need an EditorPlugin to use undoredo)

[Mickeon]

Fine by me

[akien-mga]

Thanks!